A deck/stress plate is a plate about 3" - 4" thick that replaces the cylinder head when reboring is being carried out, it has the cylinder bores in the plate cut out so a boring tool can be used this moves up and down the bore to make larger.
Some blocks move more than others when exerted to force, like wobble, the pinto is bad for this having a lot of wobble, as are a huge amount of engines, if you bore a cylinder without the deck plate it will not in a million years be correct it will actually taper slightly and be out of round, using a deck plate means 100% accuracy when the block is clamped with a head, otherwise a dead correct block without the head on that has been bored without the deck plate will move once a head is put on, and be out.
I have seen machine shops not use a deck plate it has to be said.
tabetha